Windows only: If you need frequent access to word definitions and text translation, Lingoes is a portable application that does everything from dictionary word look-ups to translation of over 80 languages.
Lingoes integrates in a variety of ways including definition-on-mouse-over—a great feature if you're learning a new language and want a quick way to check words while doing most of the translation on your own. You can also translate entire passages if you just want to pull the gist out of text without laboring through word-by-word translation.
In addition to the thorough dictionary system, Lingoes also has appendices that help with things like currency conversion, dialing codes, weights and measures, and other common conversions.
Have a favourite application or online tool for dictionary look ups, conversions, or other handy time savers? Let's hear about it in the comments. Lingoes is freeware, Windows only.